In the above-identified copending application, a method and apparatus are provided for detecting the presence or absence of metal objects, both ferrous and non-ferrous, in carriers, both metal and otherwise, that differ from the metal being detected in their magnetic properties. Additionally, certain embodiments of the invention forming the subject matter of that earlier application provided a method and apparatus capable of assigning a value to one or more ferrous metal parts within a non-magnetic housing which was not only indicative of the presence or absence of the part, but, in addition, its location.
The present application is directed to an extension of that same technology but is focused upon a particular application thereof, namely, that which is commonly known as "assembly verification".
Considering a specific industry, such as the automobile manufacturing industry, it is found that continually more operations are automated and may be formed robotically without intervention of human labor. Another recent development in the industry as a whole, which promotes the use of assembly verification techniques based on magnetic principles is the use of lighter weight materials, such as aluminum, various metal alloys and plastics, in place of heavy iron castings previously employed which masked metal components, such as ferromagnetic components, contained within the assembly as well as making the same so massive that it became energy inefficient. Additionally, the area of assembly verification has been enhanced by the recent development of powerful magnetic materials containing elements such as neodymium and boron.
Prior art that is conceivably pertinent to the area of scanning of ferromagnetic materials is contained in Tokura, et al., U.S. Pat. No. 4,677,378; Butler, U.S. Pat. No. 4,310,797; and Christian, U.S. Pat. No. 3,002,149, all of which are of record and were cited in the parent application identified above of which this is a continuation-in-part application. Both the Butler and Christian patents relate to switches in which a permanent magnet moves in response to the presence of a ferromagnetic material to close a circuit and thus provide the user with an indication that ferromagnetic material is present. There is no way, however, of assigning a value to the ferromagnetic material detected in terms of the quantity that is attracted to the magnet nor do these instruments provide any way of determining the location of the ferromagnetic component within the magnetic field. Rather, they only teach a qualitative approach, merely providing a "yes or no" answer and upon actuating a magnet into a closed position, so there is no way of ascertaining the precise position of the ferromagnetic element which has entered the magnetic field. Tools of the type disclosed in Butler and Christian while having utility in determining if ferromagnetic material is present in a non-metallic housing, have virtually no value in differentiating between several such ferromagnetic elements in a single housing or even providing quantitative results as to the characteristics of one ferromagnetic part within an assembly.
There are presently systems capable of not only recognizing ferromagnetic elements in non-metallic carriers, but also removing them. An example of such a system may be found in Garrott, U.S. Pat. No. 3,896,608. As will be appreciated by those skilled in the art, systems for removing ferromagnetic materials, such as the crop harvester disclosed in the Garrott patent, are generally ill suited for high precision scanning of assembly verifications in industrial settings.
The displacement sensor disclosed by Tokura, et al. includes first and second electrically interconnected piezoelectric elements mechanically connected to a permanent magnet and to an unmagnetized iron element, respectively. In the presence of moving ferromagnetic materials, i.e. a magnetic workpiece having projections and recesses disposed along its outer surface, the permanent magnet expands and contracts the first piezoelectric element, in contrast to the unmagnetized iron element which is relatively stationary in the presence of the workpiece, to generate output voltages representative of the frequency of the rotating ferromagnetic material. Essential to the operation of the Tokura, et al. displacement sensor is the presence of the second piezoelectric element with its unmagnetized iron element since the system looks to this second element to create the differential between the voltages generated by the two electrically-interconnected sub-assemblies, neither of which function independently of the other. Elimination of the second piezoelectric element with its unmagnetized element would render the sensor useless for its intended purpose, namely, that of responding differently and differentially to the presence of a magnetic material in close proximity thereto as the projections and recesses of the magnetic workpiece are alternately moved past the magnet and iron element.
Additionally, operation of the sensor is dependent upon movement of the recessed magnetic workpiece relative to the magnet and unmagnetized element connected to their respective piezoelectric elements. More specifically, signals are obtained from piezoelectric elements through movement thereof and, in accordance with the Tokura, et al. patent, such movement is achieved by expanding and contracting the magnetic piece relative to the first piezoelectric element. Accordingly, if the magnetic workpiece was positioned directly below the magnet and retained thereat for any sufficient period of time, the output voltage would drop off since physical movement of the first piezoelectric element would cease.
It should also be noted, that since the function of the Tokura, et al. sensor depends on motion, it enhances eddy current effects which, while not presenting a problem in determining speed of a motor, would impair the ability of that system to effect accurate assembly verification. For example, if the workpiece used in Tokura, et al. included aluminum, or other conductive metals, eddy currents, which change the intensity of the magnetic field created by the magnetic piece, would be generated as the workpiece rotated by the magnet. Such eddy currents would inevitably detract from a quantitative analysis of the workpiece giving rise to error for which the system would have to be adequately compensated.
Finally, the system disclosed in Tokura, et al. cannot be employed to precisely determine the position of even a single ferromagnetic element within the field of the permanent magnet. That is, Tokura, et al. does not suggest a process in which determined values are assigned to magnetic parts in an assembly thereof for employing comparison measuring techniques to verify the presence and location of each individual magnetic part in other like assemblies. In other words, the concept of quantitatively determining a ferromagnetic related magnitude relating to at least one ferromagnetic component in the assembly and comparing that magnitude to a reference ferromagnetic related value to determine whether at least one ferromagnetic component is missing from the inspected assembly, is not taught or suggested in the Tokura, et al. reference. It is this technique and the apparatus related thereto that makes it possible which is the cornerstone of the present invention.